|Two-variate power series.
In order to handle both variables equivalently
we maintain a list of coefficients for terms of the same total degree.
eval [[a], [b,c], [d,e,f]] (x,y) ==
a + b*x+c*y + d*x^2+e*x*y+f*y^2
Although the sub-lists are always finite and thus are more like polynomials than power series,
division and square root computation are easier to implement for power series.
|Since the inner series must start with a zero,
the first term is omitted in y.
|Produced by Haddock version 2.4.2|